Oxygen delignification of pulp in two stages with low pressure steam heating between stages

ABSTRACT

Methods for oxygen delignification of chemical pulps are disclosed including contacting the chemical pulp with oxygen first in an upstream reactor vessel and then in a downstream reactor vessel. The upstream reactor vessel is maintained at pressures of greater than about 3 bar, and the downstream reactor vessel is maintained at temperatures of between about 90 and 120° C. and corresponding predetermined pressures. Heating of the partially delignified chemical pulp between the upstream and downstream reactors is accomplished by contacting the partially delignified chemical pulp with low pressure steam.

FIELD OF THE INVENTION

The present invention relates to oxygen delignification of chemical pulpand, more precisely, to such oxygen delignification in two stages.

BACKGROUND OF THE INVENTION

Swedish developments in the bleaching of chemical pulp have increasedthe interest in extending delignification with oxygen prior to finalbleaching, in order to improve the pulp quality of totally chemical freebleached pulp, i.e. TCF-pulp (Totally Chlorine Free), and in order tofacilitate the closing of both TCF-systems and ECF-systems (ElementaryChlorine Free).

Several variations of two-stage systems for oxygen delignification arecurrently in operation. These systems use, for example, extendedretention time, re-mixing, and intermediate treatment for chemicaldistribution, all of which have become possible by use of a two-stagesystem. Both of these stages are thus carried out in upstream systems,and are pressurized to various degrees.

The following conditions have been found to be particularly advantageousfor such delignification processes:

in the first stage the chemical concentration (oxygen and liquor) shouldbe high, i.e. it has been found to be advantageous to charge all of thechemicals to the first stage. In other words, it is important tomaintain an initially high pressure in order to support the oxygenreactions.

for extended delignification in the first phase of the oxygen system itis extremely important to maintain a high temperature. If the alkalinityis sufficiently high for a good reaction to take place, no additionalchemicals need to be charged to the second stage (the final phase).

These two-stage systems have the most significant disadvantage ofrequiring much too high investment costs, but also that onlyhigh-quality intermediate pressure steam can be used for heating thepulp between the two stages.

SUMMARY OF THE INVENTION

In accordance with the present invention, these and other disadvantageshave been overcome by the discovery of a method for the oxygendelignification of a chemical pulp which comprises contacting thechemical pulp with oxygen in an upstream reactor vessel having a top anda bottom by supplying the chemical pulp to the bottom of the upstreamvessel and withdrawing a partially oxygen delignified chemical pulp fromthe top of the upstream vessel, maintaining a pressure of greater thanabout 3 bar in the upstream reactor vessel, contacting the partiallyoxygen delignified chemical pulp with oxygen in the downstream reactorhaving a top and a bottom by supplying the partially oxygen delignifiedchemical pulp to the top of the downstream reactor vessel andwithdrawing oxygen delignified chemical pulp from the bottom of thedownstream reactor vessel, maintaining the downstream reactor vessel ata temperature between about 90 and 120° C. and a correspondingpredetermined pressure, and heating the partially delignified chemicalpulp between the upstream and the downstream reactor vessels bycontacting the partially delignified chemical pulp with low pressuresteam. A preferred embodiment of the method of the present inventionincludes adding delignification chemicals to the chemical pulp prior tothe upstream reactor vessel.

In accordance with one embodiment of the method of the presentinvention, the method includes maintaining a gas space at the top of thedownstream reactor vessel and the method includes degassing thedownstream reactor vessel from the top of the downstream reactor vessel.

In accordance with another embodiment of the method of the presentinvention, the method includes recovering the heat content from theoxygen delignified pulp. Preferably, such recovery comprises flashing orheating dilution liquid therewith.

In accordance with another embodiment of the method of the presentinvention, the method includes adding hydrogen peroxide to the partiallyoxygen delignified chemical pulp prior to the downstream reactor vessel.In a preferred embodiment, the hydrogen peroxide is added in an amountof up to about 5 kg/ton of pulp. Most preferably, the hydrogen peroxideis added by means of a mixer.

A primary object of the present invention is to solve the aforesaidproblems by carrying out the two oxygen delignification stages underpressure, in that the first stage is carried out in an upstream reactorvessel and the second stage in a downstream reactor vessel. Furtherextended delignification, as well as an increase in pulp brightness, areobtained by supplying hydrogen peroxide in amounts of up to 5 kg per tonof pulp, to a mixer located prior to the downstream reactor. A minoradjustment for the increased alkali demand of the peroxide reactions canthus be made in this mixer. By means of this peroxide supply to thedownstream reactor the pulp can be delignified to a greater extent andthe brightness of the pulp can be increased prior to the final bleachingstep. In this manner, closing of the bleaching plane is furtherfacilitated.

The method according to the present invention additionally offersseveral other advantages, as follows:

no separate pump is required between the reactors;

the pulp is heated at the lowest pressure level in the system, so thatlow pressure

steam can now be used for heating the pulp;

possible admixture of additional chemicals, such as hydrogen peroxide,and adjustment of the alkali level, can take place before the secondstage in a mixer located at the top of the second reactor vessel;

due to the fact that the downstream reactor is not filled with pulp, thesystem can easily be de-aired (degassed) from the top of the downstreamreactor vessel;

the steam recovered by flashing the pulp downstream of the second stagecan be used for heating purposes, such as the heating of water;

the temperature of the pulp at the bottom of the downstream reactor canalso be lowered by dilution with colder liquid, thereby making itpossible to recover the heat which is not flashed off; and

the downstream reactor constitutes a buffer by acting both as a reactorand as a storage tower.

BRIEF DESCRIPTION OF THE DRAWING

The present invention may be more fully appreciated with reference tothe following detailed description, which, in turn, refers to thedrawing which is a schematic representation of a plant for carrying outthe method according to the present invention.

DETAILED DESCRIPTION

In the plant shown in the drawing unbleached pulp of mediumconcentration (8-20%) is pumped by a pump 1 to a mixer 2 for theadmixture of oxygen and alkali to the pulp. The pulp is then directed toa first oxygen delignification stage consisting of an upstreampressurized reactor vessel 3.

The pulp is charged at the bottom of the reactor and removed at the top.The pulp is then fed to a second oxygen delignification stage consistingof a downstream pressurized reactor vessel 4 in which the pulp issupplied at the top and removed at the bottom of the vessel. Additionalchemicals can optimally be admixed between the stages by means of amixer 9. For example, hydrogen peroxide, in an amount of up to 5 kg perton of pulp, and a small amount of alkali for adjustment of the alkalilevel, can be so added. After the second stage the oxygen delignifiedpulp is directed to subsequent processing stages 5, which can consist ofa blow tank where the pulp is flashed, and where recovered steam can beutilized for heating the process water. Stage 5 may also be a washingstage. For cooling the pulp, it is diluted at the bottom of the secondreactor by washing filtrate cooled in a heat exchanger 7. Warm water issimultaneously heated in this heat exchanger, and the resulting hotwater is used as washing liquid in the process. A container 6 isprovided for de-airing the pulp in the vessel 4.

The chemicals (oxygen and alkali) added in the mixer 2 should besufficient to constitute the principal portion of the total amountrequired. Preferably, the entire amount of chemicals has already beencharged in the first stage. Possible additional chemical charges can bemade between the stages.

In the first stage a pressure above 3 bar, preferably from about 3 to 10bar, is maintained. The temperature should be between about 75 and 100°C.

Because of the fact that the pulp is pumped upward through the firstreactor vessel 3, and is thereafter transferred to a downstream reactorvessel 4, no additional pump is required between the reactor vessels.

In the second stage a temperature of between about 90 and 120° C. ismaintained, as well as a pressure which is adapted to the temperature,which is sufficiently low to render it possible to use low pressuresteam 8 for heating the pulp, preferably at a maximum pressure of about2 bar. The pulp is preferably heated with low pressure steam between thestages at the lowest pressure level of the system.

In the reactor vessel 4 of the second stage a gas space is maintained atthe upper part of the vessel. Degassing of the system can thus becarried out at the top of the vessel. The reactions taking place withoxygen in the second stage take place only with oxygen which isdissolved in the pulp, and with the oxygen remaining after degassing.The pulp leaving the second stage will thereby contain a smaller amountof gas than would otherwise be the case and, therefore, subsequentwashing stages can operate more efficiently.

The temperature of the pulp at the bottom of the reactor vessel 4 of thesecond stage can be lowered by using a colder liquid for dilution. Inthis manner, the heat which is flashed off can be recovered.

The method of the present invention also renders it possible for thesecond vessel 4, besides having a function as a reactor for oxygendelignification, to also act as a storage tower for the pulp, therebyserving as a buffer in the system.

The two-stage system for oxygen delignification described above is amuch simpler and cheaper method of delignification. Most particularly,the economy of the operation can be improved by heating the pulp betweenthe stages with low pressure steam, instead of the medium pressure steamrequired in previous systems.

The method according to the present invention thus makes it possible tosubstantially reduce both investment and operating costs. It is aboveall now possible to save considerable energy in relation to conventionalsystem for oxygen delignification in two stages.

Although the invention herein has been described with reference toparticular embodiments, it is to be understood that these embodimentsare merely illustrative of the principles and applications of thepresent invention. It is therefore to be understood that numerousmodifications may be made to the illustrative embodiments and that otherarrangements may be devised without departing from the spirit and scopeof the present invention as defined by the appended claims.

What is claimed is:
 1. A method for the oxygen delignification of achemical pulp which comprises contacting said chemical pulp with oxygenin an upstream reactor vessel having a top and a bottom by supplyingsaid chemical pulp to said bottom of said upstream reactor vessel andwithdrawing a partially oxygen delignified chemical pulp from said topof said upstream reactor vessel, maintaining a pressure of greater thanabout 3 bar in said upstream reactor vessel, contacting said partiallyoxygen delignified chemical pulp with oxygen in a downstream reactorvessel having a top and a bottom by supplying said partially oxygendelignified chemical pulp to said top of said downstream reactor vesseland withdrawing oxygen delignified chemical pulp from said bottom ofsaid downstream reactor vessel, maintaining said downstream reactorvessel at a temperature of between about 90° C. and 120° C. and acorresponding predetermined pressure, and heating said partiallydelignified chemical pulp between said upstream and downstream reactorvessels by contacting said partially delignified chemical pulp with lowpressure steam having a pressure of 2 bar or less.
 2. The method ofclaim 1 including adding delignification chemicals to said chemical pulpprior to said upstream reactor vessel.
 3. The method of claim 1including maintaining a gas space at said top of said downstream reactorvessel, and including degassing said downstream reactor vessel from saidtop of said downstream reactor vessel.
 4. The method of claim 1including recovering the heat content from said oxygen delignified pulp.5. The method of claim 4 wherein said recovering of said heat contentcomprises flashing or heating dilution liquid therewith.
 6. The methodof claim 1 including adding hydrogen peroxide to said partially oxygendelignified chemical pulp prior to said downstream reactor vessel. 7.The method of claim 6 wherein said hydrogen peroxide is added in anamount of up to about 5 kg/ton of pulp.
 8. The method of claim 7 whereinsaid hydrogen peroxide is added by means of a mixer.